Who gets which letter

On the arrears path the danger was never the letter that went out late. It was the letter that should never have gone out at all: to a customer on the vulnerability register, to an estate, to someone who paid on Tuesday. Zero escalations is not luck. It is an exclusion list that runs before the routing, every night, in the same order.

A collections letter is the one piece of customer contact a company sends at scale to people who are, by definition, having a bad month. Most of them should get one; the routing for those is easy and boring. The whole difficulty is the rest: the accounts that must not get a letter tonight, whatever their age or balance says. A dispute that is still open. A payment plan that is being kept. A "no letters" preference that was recorded two years ago and honoured ever since. In the arrears case the discipline that held those lines was a reconciliation: every account in, every account out, exactly once, and no file to the mailhouse if the sum broke. The Eligibility Engine is that discipline written down as rules and run on a schedule — SEAL and WATCH of the Leak Ledger, pointed at harm rather than cash.

The engine is deliberately boring: no scoring model, no "propensity", no rule that depends on a rule further down. Each exclusion is a sentence a complaints handler can read and disagree with, the order is printed, and the two numbers a business could argue about — the balance under which a letter isn't worth sending, and the minimum gap between letters — are sliders, not constants. That is what makes the output trustworthy: when the engine says an account gets a formal notice, the trail says which seven rules looked at it first and why none of them fired.

# the exclusions, in order; the first match owns the account (engine.py)
def hits(a, rules_on, threshold_p, gap):
    h = []
    if rules_on["E01"] and a["deceased"]: h.append("E01")
    if rules_on["E02"] and a["vulnerable"]: h.append("E02")
    if rules_on["E03"] and a["dispute"]: h.append("E03")
    if rules_on["E04"] and a["plan"] and a["plan_kept"]: h.append("E04")
    if rules_on["E05"] and a["paid14"]: h.append("E05")
    if rules_on["E06"] and a["since_letter"] <= gap: h.append("E06")
    if rules_on["E07"] and a["pence"] < threshold_p: h.append("E07")
    if rules_on["E08"] and a["no_letters"]: h.append("E08")
    return h                      # h[0] fires; len(h) == 1 means it guarded alone

def route(a):                        # only for accounts no rule caught
    d = a["days"]
    if d < 7: return "NOT_YET_DUE"
    if d <= 21: return "A"
    if d <= 45: return "B"
    if d <= 90: return "C" if a["pence"] >= FORMAL_MIN_P else "B"   # a formal notice needs £100
    return "D"

Two things to try. Press NO EXCLUSIONS and the prevented count becomes the letters that would have gone out: every account on the vulnerability register gets a reminder, and the estate of a customer who has died gets a final notice. That is what "the routing is fine" looks like without the gate in front of it. Then switch only E06 off and watch the sole-guard column in the table below: most of the accounts it protects are protected by nothing else — a minimum gap is not politeness, it is the rule that stops the same person getting three letters in a fortnight.

The audit trail, twelve accounts

The first account each rule caught, then the first account sent to each script. Every account in the base has a line like this; a complaints handler can pull it by id.